Rho

Rho is classed as a liquid consonant (together with Lambda and sometimes the nasals Mu and Nu), which has important implications for morphology.

In polytonic orthography, a rho at the beginning of a word is almost always written ⟨ῥ⟩ (rh) with a rough breathing mark, indicating that it is voiceless.

However, this practice fell out of use over the 19th century in favour of ⟨ρρ⟩, since double rho cannot take any other combination of breathing marks.

Letters that arose from rho include Roman R and Cyrillic Er (Р).

The characters ρ and ϱ are also conventionally used outside the Greek alphabetical context in science and mathematics.